The present invention relates to a high frequency amplifier for coupling a load with variable operating conditions, particularly a load in form of the plasma to be heated by high frequency energy in an experimental reactor for nuclear fusion, using a tetrode as the amplifier tube and including a regulating circuit with a set point adjuster for regulating the anode voltage.
The operating conditions of the load are determined by the high frequency values, i.e. by the real (that is ohmic) and reactive (capacitive or inductive) components of the load resistance. In the ideal matching case, the load has a purely ohmic resistance without any reactive component.
The mismatching is definable as the standing wave ratio s, i.e. the ratio between the real, outgoing power and the reactive, returning power. In the case of equilibrium thereof s=.infin. and in the case of only outgoing power s=1.
When using high frequency energy for heating plasma in test reactors for nuclear fusion, frequencies of ion cyclotron resonance are used. These resonant frequencies in existing fusion reactors are in the range 10 to 120 MHz. Very high frequency power is required of the generators for plasma heating and at present roughly 2 MW per generator are required. Transmitting tubes are available which are fundamentally able to provide such power levels. Mainly high power tetrodes are used. However, tetrodes only permit limited power dissipation at their electrodes and the tube is destroyed if this is exceeded.
An important limit for the efficiency of such tubes is the anode power dissipation, which is largely dependent on the mismatching on the tube. This mismatching results from the fact that the reactor plasma to be heated is not constant and instead has widely varying high frequency values during heating. These are transformed by means of the antenna, the coaxial high frequency line, and the matching elements of the generator as mismatching on the tube anode. A real resistance transformed to a corresponding value is considered to be the optimum matching. In addition, the necessary frequency band width of such an amplifier in the case of constant power is made relatively high. For example, this band width is .+-.2 MHz at a 50 MHz centre frequency. There can be a mismatching of s=1.5 or higher on the connected high frequency line, so that due to the transformation in the anode circuit, the mismatching at the tube can assume values of s=2.6 or higher. There are considerable external resistance changes for the transmitting tube for the different phase positions.
The high frequency power of an amplifier stage is calculated from the difference between the power fed in I.sub.a .times.V.sub.a and the anode power dissipation. The anode current must be increased as the external resistance decreases, so that a given required power is obtained. Assuming that the anode voltage remains constant, this leads to a rise in the anode power dissipation. However, the permitted anode power dissipation is limited for each tube.